1. Field of the Invention
The present invention relates to a negative electrode active material that can absorb and emit a lithium ion, a method for producing this negative electrode active material, and a lithium ion secondary battery having a negative electrode using this negative electrode active material.
2. Description of the Related Art
With the widespread diffusion of small-sized electronic devices such as a mobile terminal in recent years, further miniaturization, weight saving, and life-elongation of devices are highly required. For these market demands, development of secondary battery is proceeding, in particular a small-sized, light weight one which can achieve a high energy density.
This secondary battery is also evaluated to apply to large-sized electronic devices such as an automobile, electricity storage systems such as a house, not only to small-sized electronic devices.
Among them, lithium ion secondary battery is greatly expected, since it is easy to achieve small-size and high capacity in the lithium ion secondary battery, and higher energy density can be obtained in the lithium ion secondary battery compared to a lead battery or a nickel-cadmium battery.
A lithium ion secondary battery described above is provided with a positive electrode and a negative electrode, a separator and an electrolytic solution. The negative electrode contains a negative electrode active material which participates in charge/discharge reaction.
As this negative electrode active material, carbon materials are widely used, whereas further improvement of a battery capacity is required due to the recent market demand.
It has been evaluated to use silicon as a negative electrode active material in order to improve a battery capacity. Because the theoretical capacity of silicon (4199 mAh/g) is larger than the theoretical capacity of graphite (372 mAh/g) by more than ten times, and therefore large improvement of a battery capacity can be expected.
The development of a silicon material as a raw material for a negative electrode active material is not limited to silicon simple substance, but alloys and compounds such as oxides are also evaluated.
Further, as the shapes of a negative electrode active material, application type which is standard in carbon materials or built-in type which is directly deposited on a current collector are evaluated.
However, when silicon is used as a main raw material for a negative electrode active material, the negative electrode active material expands/contracts during charge/discharge, and therefore breakage is liable to occur mainly in the vicinity of the negative electrode active material surface. Further, an ionic material is formed in an active material, and the negative electrode active material liable to break.
When a surface layer of a negative electrode active material break, a new surface is formed thereby, and the reaction area of an active material increases. At this time, the decomposition reaction of an electrolytic solution occurs at the new surface and the new surface is covered with a coat of a decomposition material of the electrolytic solution, and therefore the electrolytic solution is consumed. Accordingly the cycle characteristics is liable to deteriorate.
It has been evaluated various investigations heretofore regarding a negative electrode material for a lithium ion secondary battery mainly consists of silicon material or arrangement of electrodes in order to improve initial battery efficiency or cycle characteristics.
Concretely, silicon and amorphous silicon dioxide are simultaneously deposited by vapor-phase method in order to obtain excellent cycle characteristics or high safety (see patent literature 1, for example). Moreover, a particle of silicon oxide is provided with a carbon material (an electron conductive material) on its surface layer in order to obtain high battery capacity or high safety (see patent literature 2, for example).
Further, an active material containing silicon and oxygen is prepared, and an active material layer in which the oxygen ratio is high in the vicinity of the current collector in order to improve cycle characteristics and obtain high input/output characteristics (see patent literature 3, for example).
Moreover, oxygen is contained in a silicon active material, in which the mean oxygen content is 40 at % or less and is formed so as to increase the oxygen content in the vicinity of the current collector in order to improve cycle characteristics (see patent literature 4, for example).
Further, nano composite containing a Si phase, SiO2, MyO metal oxide is used in order to improve initial charge/discharge efficiency (see patent literature 5, for example).
Moreover, SiOx (0.8≤x≤1.5, a particle size range=1 μm to 50 μm) and a carbon material are mixed and fired at high temperature in order to improve cycle characteristics (see patent literature 6, for example).
Further, a molar ratio of oxygen to silicon in a negative electrode active material is set to 0.1 to 1.2 and an active material is controlled in a range of the difference between the maximum and the minimum of the molar ratio in the vicinity of the interface of the active material and a current collector is 0.4 or less in order to improve cycle characteristics (see patent literature 7, for example).
Moreover, metal oxide containing lithium is used in order to improve battery loading characteristics (see patent literature 8, for example).
Further, a hydrophobic layer such as silane compound is formed on a silicon material surface layer in order to improve cycle characteristics (see patent literature 9, for example).
Moreover, conductivity is given by using silicon oxide and forming a graphite coat thereon in order to improve cycle characteristics (see patent literature 10, for example). In patent literature 10, broad peaks appeared at 1330 cm−1 and 1580 cm−1 in a shift value of RAMAN spectrum regarding graphite coat, and their intensity ratio I1330/I1580 is in the range of 1.5<I1330/I1580<3.
Further, a particle containing a silicon microcrystalline phase dispersed in silicon oxide is used in order to obtain a high battery capacity and improve cycle characteristics (see patent literature 11, for example).
Moreover, silicon oxide in which a molar ratio of silicon atom and oxygen atom is controlled to 1:y (0<y<2) is used in order to improve overcharge/overdischarge characteristics (see patent literature 12, for example).